1. Field of the Invention
The present invention relates to novel methods and devices for preventing deposition on an optical component in an absorption spectroscopy measurement cell. The present invention also relates to measurement cells useful in absorption spectroscopy measurement. The invention further relates to apparatuses for performing an absorption spectroscopy measurement, and to semiconductor processing apparatuses.
2. Description of the Related Art
Semiconductor integrated circuits (ICs) are manufactured by a series of processes, many of which involve the use of gases. Included among such processes are etching, diffusion, chemical vapor deposition (CVD), ion implantation, sputtering and rapid thermal processing. The use of an in-line absorption spectroscopy cell to monitor impurities in such processes is described, for example, in U.S. Pat. No. 5,963,336, to McAndrew et al, the contents of which are incorporated herein by reference.
The sensitivity of detection of gas phase molecular species by absorption spectroscopy increases as the length of the light path through the sample increases, for constant pressure and concentration. The intensity of light reaching the detector is given by Beer""s Law as follows:
I=Ioxc2x7exe2x88x92xcex1cPl
where Io is the intensity of the incident radiation, xcex1 is the absorptivity, 1 is the pathlength through the sample, c is the concentration of the impurity in the sample (by volume) and P is the total pressure of the sample. For small absorptions, the amount of light absorbed is given by
Ixe2x88x92Io=xcex1cPl
In order to make l large, it is frequently impractical to place the light source and detector very far apart. Thus, xe2x80x9cfoldedxe2x80x9d light paths are often used, in which mirrors reflect the light back and forth through the sample gas many times.
The Herriott design is often preferred for tunable diode laser absorption spectroscopy (TDLAS). As shown in FIG. 1, the exemplary Herriott cell 100 uses two curved mirrors 102 mounted at opposite ends of a usually cylindrical gas sample cell 104. Other types of multi-pass cells are also known. Simple multi-pass arrangements are often used, such as described in U.S. Pat. No. 3,524,066, to Blakkan, and U.S. Pat. No. 5,173,749, to Tell et al, the contents of which patents are herein incorporated by reference. A planar polygonal multipass cell is described in U.S. Pat. No. 5,818,578, to Inman et al, the contents of which are herein incorporated by reference.
Intracavity laser absorption spectroscopy (see W. Brunner and H. Paul xe2x80x9cOn the theory of selective intracavity absorptionxe2x80x9d Optics Communications 12(3) 252 (1974)), also known as intracavity laser spectroscopy (see U.S. Pat. No. 5,723,864, to Atkinson et al) is based upon the principle of absorption of laser light within a laser cavity. Cavity ring-down spectroscopy (see A. O""Keefe et al, xe2x80x9cCavity ring-down spectrometer for absorption measurements using pulsed laser sourcesxe2x80x9d Rev. Sci. Instrum. 59(12) 2544 (1988) and U.S. Pat. No. 5,973,864, to Lehmann et al) is based upon laser light absorption within an external cavity. Both of these methods are considered sophisticated types of absorption spectroscopy. They provide very high sensitivity and rely upon the probe light beam making many passes through the cavity and therefore upon very highly reflective optics.
Various gases used in the semiconductor manufacturing processes referenced above are highly reactive and tend to form deposits on surfaces with which they come into contact, especially under conditions used in IC fabrication, such as high temperature or plasma conditions. When an in-line spectroscopic sensor is used to monitor a process in such aggressive atmospheres, deposits from the process gases tend to form on surfaces of the sensor, including, for example, on optical components such as light reflective mirrors and light transmissive windows.
Deposits formed on the optical components of a spectroscopic cell can adversely impact the sensitivity and operation of the sensor. For example, deposits formed on the reflective surfaces of the mirrors can reduce their reflectivity and hence the light intensity which reaches the detector after multiple reflections of the light beam. Likewise, the formation of deposits on the light transmissive window(s), through which the light beam enters and exits the measurement cell, reduces the light intensity reaching the detector. Such reduction in light intensity decreases the measurement sensitivity and may eventually lead to a condition in which the sensor does not function at all.
Deposits on the mirrors and light transmissive windows are conventionally removed by disassembling the sensor and mechanically/chemically cleaning the contaminated components. Such maintenance, however, is inconvenient and expensive. Avoidance or minimization of the deposits is therefore desirable.
U.S. Pat. No. 5,360,980, to Borden et al, discloses a particle sensor for monitoring the particle level of a process chamber by light scattering. To prevent contamination by corrosive or coating species in the effluent from the process, a gas purge line allows a flow of gas to purge the optical components at a flux not less than the flux of gas being removed from the process chamber in the exhaust line. Use of a purge gas with such a high flowrate is undesirable because of the increased load on the vacuum pump. Consequently, replacement of the exhaust line with one of larger diameter, and possibly replacement of a vacuum pump with one of higher capacity may be required. Additionally, the possibility of back-contamination of the process chamber is increased.
To meet the requirements of the semiconductor processing industry and to overcome at least some of the disadvantages of the related art, it is an object of the present invention to provide a novel method of and device for preventing deposition on an optical component useful in absorption spectroscopy. The problems associated with the formation of deposits on optical components, such as mirrors and light transmissive windows in absorption spectroscopy measurement cells can thereby be avoided or conspicuously ameliorated. In particular, the invention can minimize the flow of purge gas, employing a flux less than that of gas being exhausted from the process chamber. A minimum flow of purge gas can thereby be employed.
It is a further object of the present invention to provide an in-line cell useful in absorption spectroscopy and to provide an apparatus for performing an absorption spectroscopy measurement which comprise the novel device for preventing deposition.
It is further an object of the present invention to provide a semiconductor processing apparatus which comprises the inventive in-line cell.
Other objects and aspects of the present invention will become apparent to one of ordinary skill in the art on a review of the specification, drawings and claims appended hereto.
According to a first aspect of the present invention, novel methods of preventing deposition on an optical component in an absorption spectroscopy measurement cell, such as a tunable diode laser, an intra-cavity or a cavity ring-down spectroscopy cell, are provided. The inventive methods comprise performing an absorption spectroscopy measurement of a sample gas introduced into the cell, and introducing a flow of purge gas from a purge gas inlet pipe across a critical surface of the optical element at a velocity effective to prevent deposition on the critical surface. The gas inlet is disposed adjacent said critical surface.
According to a further aspect of the invention, devices for preventing deposition on an optical component useful in an absorption spectroscopy measurement cell are provided. The inventive devices comprise a purge gas inlet pipe for introducing a flow of purge gas across a critical surface of the optical element at a velocity effective to prevent deposition on the critical surface. The gas inlet is disposed adjacent said critical surface.
According to a further aspect of the invention, measurement cells useful in absorption spectroscopy measurement are provided. The measurement cells comprise, in addition to a device for preventing deposition as described above, a sample gas inlet, a sample gas outlet, a sample region, a light entry port and a light exit port being the same or separate ports. Each port is in optical communication with the sample region and contains a light transmissive window.
In accordance with a further aspect of the invention, apparatuses for performing an absorption spectroscopy measurement are provided. The apparatuses comprise the a measurement cell as described above, a light source for generating a light beam which passes through the light entry port into the cell, and a detector for measuring the light beam exiting the cell through the light exit port.
In accordance with a further aspect of the invention, semiconductor processing apparatuses are provided. The apparatuses comprise a semiconductor processing apparatus comprising a substrate processing chamber and an exhaust line connected thereto, and an apparatus for performing an absorption spectroscopy measurement as described above.
The invention is particularly applicable to in-situ absorption spectroscopy measurements which are useful, for example, to accurately and sensitively measure the concentration of gas phase molecular impurities, such as, e.g., methane, moisture (water vapor) and carbon dioxide, in a sample. Through the invention, the surfaces of optical elements, such as mirrors and light transmissive windows, can be maintained in a deposit-free state.
In the case of mirrors employed in absorption spectroscopy measurements, the invention ensures that reflectivity of the surface of the mirror is not reduced due to deposits formed thereon. Similarly, in the case the surface of the optical component belongs to a light transmissive window, the invention ensures that the light transmissive characteristics of the window are not degraded.
The invention has particular applicability to the prevention of deposition on optics used for in-situ measurements of gas composition, such that the clean gas flow is directed over critical surfaces of the optics and the flow of clean gas is minimized by appropriate design of the gas inlet and control of the gas velocity.